Technical Field
The invention relates generally to precision measurement instruments, and particularly to absolute position encoders.
Description of the Related Art
Various optical, capacitive, magnetic and inductive transducers, and movement or position transducers are available. These transducers use various geometric configurations of a transmitter and a receiver in a read head to measure movement between the read head and a scale. Inductive sensors are known to be one of the sensor types that is most immune to contamination by particles, oil, water, and other fluids. U.S. Pat. No. 6,011,389 (the '389 patent), which is hereby incorporated herein by reference in its entirety, describes an induced current position transducer usable in high accuracy applications. U.S. Pat. Nos. 5,973,494 and 6,002,250, which are each hereby incorporated herein by reference in their entireties, describe incremental position inductive calipers and linear scales, including signal generating and processing circuits. U.S. Pat. Nos. 5,886,519, 5,841,274, 5,894,678, 6,400,138, and 8,309,906, which are each hereby incorporated herein by reference in their entireties, describe absolute position inductive calipers and electronic tape measures using the induced current transducer. As described in these patents, the induced current transducer may be readily manufactured using known printed circuit board technology.
Different implementations of the induced current transducer (and other types of transducers) may be implemented as either incremental or absolute position encoders. In general, incremental position encoders utilize a scale that allows the displacement of a read head relative to a scale to be determined by accumulating incremental units of displacement, starting from an initial point along the scale. However, in certain applications such as those where encoders are used in low power consumption devices, it is more desirable to use absolute position encoders. Absolute position encoders provide a unique output signal, or combination of signals, at each position (of a read head) along a scale. They do not require continuous accumulation of incremental displacements in order to identify a position. Thus, absolute position encoders allow various power conservation schemes, amongst other advantages. In addition to the patents referenced above, U.S. Pat. Nos. 3,882,482, 5,965,879, 5,279,044, 5,237,391, 5,442,166, 4,964,727, 4,414,754, 4,109,389, 5,773,820 and 5,010,655, disclose various encoder configurations and/or signal processing techniques relevant to absolute encoders, and are each hereby incorporated herein by reference in their entirety.
The terms “track” or “scale track” as used herein generally refer to a region of the scale or scale pattern that extends along the measuring axis direction and has an approximately constant width and location along the direction transverse to the measuring axis. A scale track generally underlies and is aligned with a particular set of detectors that is guided along the measuring axis direction. The detectors respond to a pattern of scale element(s) in the underlying scale track to generate position signals that depends on the detector position along the track.
A common technique for encoding the absolute (ABS) position into an encoder is to use two encoder tracks of slightly different spatial wavelengths. For any two spatial wavelengths λ1 and λ2 that are very close, an ABS beat wavelength is defined as follows:
                              λ          ABS                =                                            λ              1                        ⁢                          λ              2                                                          λ              1                        -                          λ              2                                                          [        1        ]            
The ABS beat wavelength, which is a longer synthetic wavelength based on λ1 and λ2, can be used to determine a relatively coarse resolution synthetic wavelength position and approximately constitutes the ABS measuring range of the encoder. To achieve a long ABS measuring range, λ1 and λ2 are typically very similar values. For example, the two wavelengths used in one exemplary encoder are λ1=5.4 mm and λ2=5.268 mm, which provide an ABS range (ABS beat wavelength) of λABS=216 mm.
A typical method for choosing the two wavelengths is to set an integer number n of coarse wavelengths λ1 in the desired ABS range (≈λABS), then calculate the value of fine wavelength λ2 as follows:λABS=nλ1  [2]
                              λ          2                =                              λ            1                    ⁢                      n                          n              +              1                                                          [        3        ]            
In the exemplary encoder described above where λ1=5.4 mm and λ2=5.268 mm, λ1 and λ2 are calculated based on choosing n=40.
It is known to configure encoder tracks and corresponding detectors in order to generate signals that can be processed to determine the spatial phase (or position) of a detector within any given wavelength or period of either/both of the tracks that have the spatial wavelengths λ1 and λ2. Given such spatial phase information, and or position information, the absolute spatial phase and or absolute position within the synthetic ABS beat wavelength λABS may be determined according to known methods. Use of λ1 and λ2 that are very similar to each other, however, may lead to certain accuracy, resolution, and/or range limitations due cross-talk error and/or other difficulty in signal isolation, especially in compact low power encoders.
Users desire improvements to the known encoder systems outlined above in order to provide improved combinations of compact size, measuring range, resolution, low power, low cost and robustness to contamination. Configurations for absolute encoders that provide such improved combinations would be desirable.